Audio frequency limiting device



n 1936- J. HAMMQND, JR,

AUDIO FREQUENCY LIMITING DEVICE Filed March 6, 1933 NOI-LOW I WW IN VENTOR I fi 4 SW; A TTORNEX 8 Patented June 2, 1936 UNITED STATES PATENTOFFICIE AUDIO FREQUENCY LIMITING DEVICE John Hays Hammond, Jr.,Gloucester, Mass. Application March 6, 1933, Serial No. 659,665

9 Claims. (01. 179-1) This invention relates to sound reproducing orrecording systems and more particularly to an improved means forpreventing overloading of an electro-dynamic sound reproducer orrecorder,

5 such as a loud speaker, phonograph record engraving tool, or the like.

In one embodiment of the invention, the field windings of the soundreproducing or recording device are energized by a current which isvaried 10 as a function of the input tone current. This may beaccomplished by causing an excess in input volume, above a predeterminedvalue, to cut down the field current to such a degree that the responseof the device is definitely limited. The invention accordingly preventsthe response from reaching the point where distortion or chatteringwould occur, for example, by reason of the moving element coming intocontact with the mechanical stop members.

s The invention is also applicable to the cutting of phonograph recordsand prevents the records from being overcut as a result of loud inputvolume.

The invention also consists in certain new and original features ofconstruction and combinations of parts hereinafter set forth andclaimed.

Although the novel, features which are be- .lieved to be characteristicof this invention will be particularly pointed out in the claimsappended hereto, the invention itself, as to its objects and advantages,the mode of its operation and the manner of its organization may bebetter understood by referring to the following descrip-' tion taken inconnection with the accompanying drawing forming a part thereof, inwhich Fig. 1 illustrates diagrammatically one form of circuit embodyingthe present invention; and

Fig. 2 is a curve showing the relation between the impressed voltage andthe motion of the moving elements.

In the following description and in the claims, parts will be identifiedby specific names for convenience, but they are intended to be asgeneric in-their application to similar parts as the art will permit.

Referring more particularly to the drawing, a light source I isprovidcd,'- the light from which is focused by means of alens through anaperture l2 in an aperture plate l3 upon the sound record of a movingphotographic film I4. The light which passes through this film isfocused by a second lens |5 upon a photoelectric cell I6. This cell isconnected through a battery I! to the primary of a transformer l8.

A phonographic pickup device 22 of well known form may be provided foruse with a disk record. A microphone 24 may also be provided for soundrecording purposes and may be connected to the primary of a transformer25. The secondary of the transformer l8, the winding ofthe pickup device22, and the secondary of the transformer 25 may be selectively connectedacross the input of an amplifier 21 by a three position switch 2| havingcontacts I9, 23 and 26 which are respectively connected thereto.

. The amplifier 21 may, comprise, for example, a space dischargeamplifying means of standard form, having one or more stages ofamplification as desired. The output circuit of the amplifier 21includes the primary of the transformer 3|, the secondary of which maybe selectively connected across the armature or moving coil 32 of aloudspeaker 33 or across the armature coil 34 of a. phonograph recordengraving tool 35 by means of contacts 36 and 38 of a double-poledouble-throw switch 31.

The output circuit of the amplifier 21 also includes the primary of atransformer 4|. The

secondary of this transformer is connected in a circuit including abattery 42, rectifier 44 and resistance 45, which may be shunted by acondenser 46 adapted to control the time constant of said circuit. Thebattery 42 is connected to oppose the fiow of current through therectifier 44 so that no current will be passed thereby until theimpressed voltage exceeds the voltage of said battery. The resistance 45is connected between the filament and grid of a space discharge device43 so as to control the grid potentialthereof.

The plate circuit of the space discharge device 43 includes a secondary41 of a transformer 52, which serves as a source of plate potential, andthe field winding 5| of loud speaker 33 or the field winding 53 of theengraving tool 35, said windings 5| and 53 being connected respectivelyto contacts 52 and 54 of the switch 31, whereby they -may be alternatelyrendered operative. The primary of transformer 52 is connected to asource of alternating current supply 55. A second secondary 48 oftransformer 52 serves to energize the cathode of the device 43. Acondenser 56 may be connected across the windings 5| and 53 to controlthe operating characteristics thereof.

In the operation of the above described system the switch 2| is throwninto an engagement with the contact 19 when it is desired to use thesound film l4. When the phonograph pickup 22 is to be used, the switch2| is thrown into engagement with the contact 23, and when it is desiredto use within normal limits.

the microphone 24 the switch is thrown into engagement with contact 26.In any case the energy is fed to the amplifier 21 where it is amplified.

and supplied to the transformer 3|.

If the system is to be used for sound reproduction the switch 31 isthrown into position to engage contacts 36 and 52'. In this case energyfrom the secondary of the transformer 3| is supplied to the moving coil32 of the loud speaker 33 and the field winding 5| of this loud speakeris energized by the uni-directional space current of the device 43.

Part of the output energy of the amplifier 21 is supplied to thetransformer 4| and causes a voltage to be impressed upon the circuitincluding the rectifier 44. No current will pass the rectifier 44,however, until the impressed voltage exceeds the voltage of the battery42. When the input current is such that the impressed voltage exceedsthis value, unidirectional current will flow through the resistance 45in the direction of the arrow, thus building up a potential differenceacross this resistance which is dependent upon the excess of input tonecurrent above the predetermined value. This causes a negative potentialto be placed on the grid of the device 43 which is proportional to thisexcess of tone current, thereby increasing the impedance of this deviceand decreasing the space current passed thereby. In this way themagnetization current fed to the field winding 5| is decreased, therebydecreasing the motor efiiciency'of' the loud speaker and cutting downthe response thereof. Obviously additional grid biasing means may beemployed if desired, the amount of biasing potential depending upon thecharacteristics of the device 43.

The various constants are so selected that the normal operation of theloud speaker is not interfered with. But when the input signal isincreased to a point such that distortion or rattling would occur, thedevice 43 functions to reduce the current in the field winding, thusreducing the motion of the moving element so as to keep it of such valuethat the current variations in reslstance 45 are caused to correspond tothe average energy level of the signal over a definite time intervalcorresponding, for example, to a syllable or a musical note, asdistinguished from the instantaneous energy level of the sound waves. Itis to be understood that the time response of the circuit includingrectifier 44 and resistance 45 may be controlled by suitable adjustmentof condenser 46.

From the above it will be clear that the average energy level iscontinually changing as the signal changes although it does not changeas rapidly as the instantaneous energy level. The average energy level,since it is continually changing, may be called a moving average of thesignal energy.

Figure 2 shows a curve illustrating a preferred relationship between theimpressed voltage and the motion of the moving element. It is seen fromthis curve that as the impressed voltage increases the motion of themoving element increases uniformly to the point 51. Beyond that point,as the impressed voltage increases, the increase in motion becomes lessand less and never reaches a point such that distortion occurs, forexample, as a result of the moving elemen striking the mechanical stops.

If the system is to be used for sound recording the switch 31 is throwninto engagement with contacts 38 and 54, thus connecting the armaturecoil 34 to the secondary of the transformer 3| The condenser 46 may beand the field coil 53 in the plate circuit of the device 43. Theoperation of the system for sound recording is the same as that justdescribed in connection with the loud speaker, that is, if thetone'current increases beyond a predetermined value, the magnetizationcurrent in the field winding 53 will be relatively decreased, thuspreventing the engraving tool 35 from overcutting the record.

Only so much of the circuits have been shown as is necessary to anunderstanding of the invention. The various details are understood to beof standard form and will be readily apparent to a person skilled in theart. The invention is not to be limited to any specific construction,but may be embodied in various forms without departing from the spiritthereof. It is only to be limited in accordance with the followingclaims when interpreted in view of the prior art.

What is claimed is:

1. The method of operating an electro-dynamic sound reproducing orrecording device having a variable magnetic field and exciting meanstherefor which comprises utilizing an increase in average input soundenergy above a predetermined 1..

minimum to decrease the field excitation whereby the response of saiddevice is prevented from exceeding a predetermined value.

2. The method of operating an electro-dynamic sound reproducing orrecording device having armature and field windings which comprisescausing the field excitation to vary inversely as the average armatureexcitation when the average ar mature excitation exceeds a predeterminedvalue whereby the response of said device is prevented from exceeding apredetermined value.

3. In a sound reproducing or recording system having an electro-dynamicreproducing or recording device provided with a field winding,

means to energize said winding, and means dependent upon the averageinput sound energy to decrease said energization when said average inputenergy exceeds a predetermined value.

4. In a sound reproducing or recording system having an electro-dynarnicreproducing or recording device provided with a. field winding, means toenergize said winding, a space discharge device adapted to control saidenergization, and means to increase the impedance of said spacedischarge device when the input sound energy exceeds a predeterminedvalue whereby to decrease said field energization.

5. A sound reproducing or recording system comprising a source of soundenergy, amplifying means therefor, an electro-dynamic device havingarmature and field windings, means energizing said armature windings inaccordance with said amplified energy, means to energize said fieldwindings, and means to decrease said last mentioned energization whenthe average level of said sound energy exceeds a predetermined valuewhereby to limit the response of said device.

6. A sound reproducing or recording system comprising a source of soundenergy, amplifying means therefor, an electro-dynamic device havingarmature and field windings, means energizing said armature windings inaccordance with said amplified energy, means to energize said fieldwindings, and means to decrease said last mentioned energization whenthe average level of said sound energy exceeds a predetermined valuecomprising a variable impedance device, a resisteluding a rectifier anda source of potential adapted to oppose the flow of current in saidcircuit, and means to impress on said rectifier a potential dependentupon the input sound energy, an increase in potential drop across saidresistance being 5 adapted to increase the impedance of said device.

7. In a sound recording system, a recording device having an armaturecontrolled by the sound energy and a field energized from an independentsource, and means controlled by the average level of the sound energyfor reducing the field energizatlon when said average level exceeds apre determined value to prevent overcutting of adjacent soundtracks.

8. In a phonograph system, a pick-up device .by, a field in saidtranslating device, exciting means therefor, and means controlled by theaverage energy from said pick-up device for reducing the fieldexcitation when said average energy exceeds a predetermined value.

9. In a sound record system, 'a pick-up device for sound energy, atranslating device fed thereby, a field in said translating device,exciting means therefor, and means controlled by a moving average of theenergy from said pick-up device for reducing the field excitation whensaid average energy exceeds a predetermined value.-

JOHN HAYS HAMMOND, an.

